CYCLOPROPYL CONJUGATION IN OLEFINIC ESTERS . CONFORMATIONAL EFFECTS ON ULTRAVIOLET ABSORPTION

The ultraviolet absorption spectra of 17 cyclopropylacrylic esters and their respective models are reported. The effect of substitution on the spectral changes, ranging from a bathchromic effect of 29 mμ to a hypsochromic effect of 3 mμ, is evaluated in terms of the structure and population of possible conformers. The disappearance of a bathochromic effect in ester 15 is interpreted as support for the existence of a limiting geometrical requirement for cyclopropyl conjugative orbital overlap. Comparison of shielding effects on β substituents corroborates this interpretation. The magnitude of vicinal coupling constants indicates that cyclopropylacrylic esters populate the bisected conformation to a larger extent than corresponding vinylcyclopropanes. © 1968, American Chemical Society. All rights reserved.